Storage of solar energy remains a challenge that has not yet been satisfactorily resolved. This is a pivotal problem because long-term viability of solar energy requires storage. While the sun could potentially provide most of the energy the world needs, the fact that it is only available when there is sunlight makes the sun an inconvenient and unreliable source of energy. Finding a reliable and an effective way to store energy from the sun is important because the sun is the most abundant and cleanest source of energy available to humanity. An effective storage solution for solar energy would reduce reliance on fossil fuel sources—thus reducing the harmful effects of these fuels on the environment and from contributing to climate change.
The simplest approach to storing solar energy today is to charge banks of batteries with electrons (charge) liberated during the photovoltaic process. This approach, however, is limited by the amount of charge that can be stored in the battery, the spontaneous and unavoidable discharge of it, and by the relatively short charge retention period of the battery. In general, only a limited number of low power applications can rely entirely on battery storage; and even then, the batteries must be periodically recharged. Furthermore, the current chemistry of batteries is environmentally unsafe and not sustainable. Some batteries, for example those that include reactive alkali elements such as lithium, potassium, or sodium in them, can explode if not properly packaged. The very properties that make them ideal as elements of energy storage devices, also make them dangerous. Disposal of used batteries is often problematic because of the chemical toxicity of the waste.
Although several other methods have been investigated for solar energy storage, the most viable for large-scale storage usually take one of two paths to achieving sensible heat storage—where conversion of solar energy to thermal energy leads to a temperature difference between the thermal storage medium that receives it and the ambient. In one alternative of the process, a solar energy collection loop and a storage loop are usually set up. The collection loop may be comprised of an apparatus that harvests solar energy with a receiver that has a fluid to which the energy is transferred; this fluid flows in a closed loop established between the receiver and a heat exchanger. A second fluid flows in another loop that connects the same heat exchanger and a storage vessel filled with a large volume of a second fluid. The two fluids do not touch or mix but exchange energy through the heat exchanger. In this kind of system, thermal energy delivered to the receiver is absorbed by the first fluid and then transferred to the second through the heat exchanger; the large volume of the second fluid serves as the storage medium. The fluid in the storage vessel usually has a high thermal capacity; molten salt, among several others, is usually a good choice. Properly insulated salt can retain (store) heat for long periods. Thermal energy stored in this manner can be used to convert water to supersaturated steam, which can drive a turbine to generate electricity.
In the second alternative path for storing sensible heat, the first loop in the system containing heat transfer fluid is eliminated. Instead, salt is directly used to absorb the solar energy at the receiver. The salt therefore serves both as the heat transfer fluid as well as the storage medium. Elimination of the heat transfer fluid improves overall efficiency of the power generation process since losses at the first heat exchanger, which is no longer needed, are avoided.
Today, there are a number of solar energy plants installed across the world that utilize molten salt thermal storage to augment generation capacity when there is no sun. These plants are based on concentrated solar power (CSP), where sunlight is first concentrated at a receiver through which a heat-transfer fluid of one kind or another is flowing. Heat from such plants can be utilized directly to generate electricity as previously described. The process can be carried out in real time when the sun is still up or it can be performed after hours by using stored heat. In either case, CSP plants can be more efficient than photovoltaic power plants. The downside of CSP plants augmented with molten salt storage is that salt is corrosive. Storage vessels, piping, and any components made from metal parts are susceptible to corrosion. The damage incurred leads to frequent repairs, and to increased maintenance costs. Furthermore, because of high initial investment required to construct the infrastructure of a CSP plant, these systems only make economic sense if they are owned by large-scale utility plant operators.
Recently, microparticles have been proposed and investigated as possible heat transfer or storage media, for example in the following references.    1. Z. Ma, M. Mehos, G. Glatzmaier, and B. B. Sakadjian, “Development of solid particle thermal energy storage for concentrating solar power plants that use fluidized bed technology,” Energy Procedia, 69 1349-1359, (2015).    2. C. K. Ho, J. Christian, J. Yellowhair, S. Jeter, M. Golob, C. Nguyen, K. Repole, S. Abdel-Khalik, N. Siegel, H. Al-Ansary, A. El-Leathy, and B. Gobereit, “Highlights of the high-temperature falling particle receiver project: 2012-2016,” AIP Conference Proceedings, 1850 030027 (2017).    3. Z. Ma and R. Zhang, “Solid particle thermal energy storage design for fluidized-bed concentrated solar power plant”, US Patent Application, US 2013.0255667 A1, Oct. 3, 2013.
Until now, all previous work reported on use of such microparticles for heat transfer or storage has exclusively relied on high tower solar receiver architectures generally used in CSP systems. In this type of architecture, a solar field full of mirrors surrounds a tall tower, on top of which is located a receiver; the mirrors are situated in such a way that collected sunlight is directed and focused onto the elevated receiver in the tower where a heat transfer fluid is flowing, as previously described. When microparticles are substituted for the heat transfer fluid, a thin sheet of such particles is caused to fall by gravity across the receiver aperture. At the instant that the particles are falling through the aperture, they are exposed to the intense concentrated sunlight that is focused there; they absorb the thermal energy and, when they reach the bottom of the tower, are retrieved and stored. It is in this manner that heat transfer and storage are accomplished. To date, there are no installed commercial systems based on this approach. Despite limitations associated with fluid media and current difficulties with microparticles, thermal energy storage in the form of sensible heat has potential to offer a solution around solar intermittence. What is required is a robust and low-cost method that does not use thermal fluids, but overcomes the difficulties associated with the microparticles.